1. Field of the Invention
The present invention relates to devices for inspecting sealed articles of luggage by passing X-ray radiation therethrough.
2. The Prior Art
Luggage inspection apparatus commonly employs a radiation source such as an X-ray generator which produces upon a fluorescent screen a shadow image which can be read directly or indirectly. The device has a lead-lined housing which protects the operators and environment against any escape of radiation. In one variety of inspection apparatus a monitoring duct extends parallel to the screen. A test object is placed within the duct and the duct is closed off by a moveable shield during irradiation of the article. The shield is moved with the assistance of a motor drive arrangement.
U.S. Pat. No. 3,678,278 discloses a device for luggage inspection wherein forward and backward movement of a shield is effected by a drive mechanism having a chain driven by a sprocket wheel on the output shaft of a conventional motor. A slip clutch is provided to prevent possible injury to persons reaching into the monitoring duct. Such drive system requires specially designed wearing parts in the transmission as well as parts requiring special maintenance. In any event these parts reduce the reliability of the installation. Variation in speed of the screen to provide a quick start up with slowing toward the end of the movement in both forward and reverse directions is difficult to obtain with such a drive.
Thus an object of the present invention is to provide a largely maintenance-free, easily controllable motor drive system for the moveable screen of a luggage inspection apparatus, the apparatus being completely protected from escape of X-rays. In accordance with the invention, a linear motor system is employed. As those skilled in the art will appreciate, a linear motor has only a single moving part, either an electromagnetic metal rail or a stator, one of which is linearly displaced by an electromagnetic reaction in the rail to a traveling magnetic field developed by the stator. A linear motor is particularly compact and is readily adjustable both for speed of displacement and for thrust force developed by the motor.